1. Field of the Invention
This invention relates generally to the dispersing of liquids into fluidized solids. More specifically this invention relates to a method and apparatus for dispersing a hydrocarbon feed into a stream of fluidized particles.
2. Description of the Prior Art
There are a number of continuous cyclical processes employing fluidized solid techniques in which carbonaceous materials are deposited on the solids in the reaction zone and the solids are conveyed during the course of the cycle to another zone where carbon deposits are at least partially removed by combustion in an oxygen-containing medium. The solids from the latter zone are subsequently withdrawn and reintroduced in whole or in part to the reaction zone.
One of the more important processes of this nature is the fluid catalytic cracking (FCC) process for the conversion of relatively high-boiling hydrocarbons to lighter hydrocarbons boiling in the heating oil or gasoline (or lighter) range. The hydrocarbon feed is contacted in one or more reaction zones with the particulate cracking catalyst maintained in a fluidized state under conditions suitable for the conversion of hydrocarbons.
It has been found that the method of contacting the feedstock with the catalyst can dramatically affect the performance of the reaction zone. Modern FCC units use a pipe reactor in the form of a large, usually vertical, riser in which a gaseous medium upwardly transports the catalyst in a fluidized state. Ideally the feed as it enters the riser is instantaneously dispersed throughout a stream of catalyst that is moving up the riser. A complete and instantaneous dispersal of feed across the entire cross section of the riser is not possible, but good results have been obtained by injecting a highly atomized feed into a pre-accelerated stream of catalyst particles. However, the dispersing of the feed throughout the catalyst particles takes some time, so that there is some non-uniform contact between the feed and catalyst as previously described. Non-uniform contacting of the feed and the catalyst exposes portions of the feed to the catalyst for longer periods of time which can in turn produce overcracking and reduce the quality of reaction products.
It has been a long recognized objective in the FCC process to maximize the dispersal of the hydrocarbon feed into the particulate catalyst suspension. Dividing the feed into small droplets improves dispersion of the feed by increasing the interaction between the liquid and solids. Preferably, the droplet sizes become small enough to permit vaporization of the liquid before it contacts the solids. It is well known that agitation or shearing can atomize a liquid hydrocarbon feed into fine droplets which are then directed at the fluidized solid particles. A variety of methods are known for shearing such liquid streams into fine droplets.
Another useful feature for dispersing feed in FCC units is the use of a lift gas to pre-accelerate the catalyst particles before contact with the feed. Catalyst particles first enter the riser with zero velocity in the ultimate direction of catalyst flow through the riser. Initiating or changing the direction of particle flow creates turbulent conditions at the bottom of the riser. When feed is introduced into the bottom of the riser the turbulence can cause mal-distribution and variations in the contact time between the catalyst and the feed. In order to obtain a more uniform dispersion, the catalyst particles are first contacted with a lift gas to initiate upward movement of the catalyst. The lift gas creates a catalyst pre-acceleration zone that moves the catalyst along the riser before it contacts the feed. After the catalyst is moving up the riser it is contacted with the feed by injecting the feed into a downstream section of the riser. Injecting the feed into a flowing stream of catalyst avoids the turbulence and back mixing of particles and feed that occurs when the feed contacts the catalyst in the bottom of the riser. A good example of the use of lift gas in an FCC riser can be found in U.S. Pat. No. 4,479,870 issued to Hammershaimb and Lomas.
There are additional references which show the use of a lift gas in non-catalytic systems. For example, in U.S. Pat. No. 4,427,538 to Bartholic, a gas which may be a light hydrocarbon is mixed with an inert solid at the bottom part of a vertical confined conduit and a heavy petroleum fraction is introduced at a point downstream so as to vary the residence time of the petroleum fraction in the conduit. Similarly, in U.S. Pat. No. 4,427,539 to Busch et al., a C4 minus gas is used to accompany particles of little activity up a riser upstream of charged residual oil so as to aid in dispersing the oil
U.S. Pat. Nos. 5,554,341; 5,173,175; 4,832,825 and 3,654,140 all shows the use of radially directed feed injection nozzles to introduce feed into an FCC riser. The nozzles are arranged in a circumferential band about the riser and inject feed toward the center of the riser. The nozzle arrangement and geometry of the riser maintain a substantially open riser cross-section over the feed injection area and downstream riser sections. The angled feed nozzles are typical of those used to inject feed or other fluids at an intermediate portion in the riser conduit. The angled feed injectors present a number of problems for the operation of the risers. The nozzles typically extend away from the wall of the riser and into the flow path of the catalyst. Passing particles over the nozzles at high velocity can result in erosion. The nozzle protrusion can also result in quiescent zones that promote backmixing and provide sites for coke build-up to begin. The protrusion of the feed injectors can provide such zones by protecting coke from the natural erosion action of the flowing catalyst which would otherwise eliminate the coke from these sites. Excessive coke buildup can upset the hydraulic balance in a unit to the point where it is eventually forced to shut down. The processing of heavier feeds such as residual hydrocarbons can exacerbate coke production problem due to their higher coking tendencies.
An obvious solution to the problem of nozzle protrusion would be to recess the nozzles completely into the wall of the riser and thereby remove them from the catalyst flow path. This solution is not satisfactory since the feed injector tips are specifically designed to provide a relatively uniform coverage of the hydrocarbon feed over the cross-section of the riser by expanding the pattern of feed injection as it exits from the nozzle. Completely recessing the tips of the injector nozzles within the wall of the riser disrupts the ability to obtain a spray pattern over the majority of the riser cross-sectional area
It is an object of this invention to more uniformly distribute catalyst and oil over the cross-section of the riser.
It is another object of the invention to reduce areas of local variation in particle density to improve oil penetration into the particles.
It is a further object of the invention to minimize areas of backmixing and quiescence around the feed injectors that can lead to coke formation.
These objects are achieved by providing a hydrodynamic mixing zone where a plurality of feed injectors circle an intermediate portion of a contacting conduit to inject a feed into a flowing stream of particulate material. The hydrodynamic zone is also referred to as the injector zone. The invention locates the outlets of the feed injector nozzles in a shelf from which the tips of the nozzles protrude. The shelf is formed by an abrupt change in the diameter of the conduit relative to the adjacent upstream portion of the conduit. This divergence in the diameter of the conduit locates the protruding tips of the feed injectors outside of the direct flow path of the passing particulate material and maintains active and flowing particles in the regions immediately upstream and downstream of the injector tips. The shelf thereby improves the hydrodynamics in the contacting zone by eliminating the deleterious effects of the previous protrusion of the nozzles into the particle flow without recessing the nozzles into the wall of the contacting conduit. The invention thereby reduces any non-uniformity in the mixing of the particles and feed and by eliminating sites with a high potential for backmixing of the feed with the particles.
The shelf can be part of a normal transition zone that increases the size of the riser to provide a larger riser cross-sectional area. The larger cross sectional area is usually necessary to accommodate a volumetric expansion of the feed. This expansion of the feed is sometimes referred to as a molar expansion. The injectors normally direct the incoming feed at a downstream angle with respect to the particle flow. Tapering the shelf so that it provides an angled surface between the smaller upstream diameter and larger downstream diameter of the riser further reduces any quiescent area for backmixing or coke initiation. Locating the tips of the upstream directed feed injectors about the angled shelf section virtually eliminates the quiescent areas that were sites for riser coking. This uninterrupted flow path replenishes particles and erodes away coke in the dense form downstream of the initial feed injection point. This invention is particularly suited for small diameter contacting conduits where the nozzle projection can have the most disrupting influence on the particle and feed flow through the conduit.
This invention can further reduce quiescent areas by contouring profile of the contacting conduit in the location of feed injection to more actively suite the specific spray pattern of the injectors. The injectors will often create a planar spray pattern that extends horizontally over the contacting conduit in a fan shaped pattern. The fan-shaped spray stream from several injectors will collide as they meet each other to form a polygon. Where the outer edges of each injection nozzle spray pattern project in a line to the adjacent injector, the polygon pattern will have a number of sides equal to the number of injectors. Areas outside the polygon pattern, but inside the typically circular cross-section of the contacting conduit can account for 10-20%, or more, of the conduit area that is not fully utilized for contacting. In accordance with this invention, the areas to the outside of the spray pattern, but within the circular cross-section of the contacting conduit may be blocked or filled in to eliminate potentially quiescent areas between the injector nozzles. Molding of a castable or pneumatically applied refractory lining to the specific contour of the spray nozzles can provide a satisfactory filler material.
Whether used with or without a contoured lining, the overall width of the injector zone is kept relatively narrow. The width of this zone will usually not exceed twice the diameter of the nozzle that provides the injector tip and, more typically, will have a total width that approximates the nozzle size.
Accordingly, within a method embodiment, this invention includes the mixing of fluidized particles with a fluid feed stream comprised of hydrocarbons to produce a dense bed of fluidized particles. To produce the dense bed of fluidized particles, the fluidized particles and a fluidizing medium are combined in an upstream section of a contacting conduit. The dense bed of fluidized particles passes downstream in the contacting conduit through an injector zone that is defined by a circumferential band of the conduit that diverges the diameter of the conduit relative to the adjacent upstream portion and that positions a plurality of discrete feed injection outlets at the wall. At least a portion of a nozzle that provides the feed injection outlet protrudes from the wall of the conduit and injects feed at an angle relative to the conduit axis into a downstream section. The protrusion of the nozzle from the wall of the conduit does not extend into an axial projection of the inner conduit wall that extends downstream from the starting point of the diverging conduit diameter. The dense bed of fluidized particles is passed downstream from the injector zone to the downstream section of the conduit that provides a less divergent diameter interior immediately downstream of the injector zone. The feed and particles are then contacted downstream of the feed injection outlets to produce a mixture of contacted feed and particles. The mixture of contacted feed and particles is then passed to a separation zone for separation of the contacted feed from the contacted particles.
In an apparatus embodiment, this invention is a contacting conduit for contacting catalyst with an at least partially liquid phase fluid. Preferably the contacting conduit is vertically oriented. The contacting conduit is elongated and has both an upstream and a downstream end. The upstream end of the contacting conduit defines a particle inlet for adding particles and a fluidizing inlet to inject a fluidizing medium and to produce a dense particle bed. Between the upstream and downstream ends of the contacting conduit is a narrow band that defines a discontinuous increase in the inside diameter of the conduit from the upstream to the downstream ends of the conduit and thus divides the conduit into upstream and downstream sections. Circling the conduit and fixed with respect to the band, a plurality of feed injectors define outlet nozzles that extend from the inside wall of the riser and remain outside the projection of a surface projected along the axis of the conduit from the inner circumference of the upstream conduit at its junction with the band. And at the other end of the vertical contacting conduit is the downstream end that defines a fluid outlet.
Additional objects, embodiments and details of this invention can be obtained from the following detailed description.